1. Field of the Invention
The present invention relates to a wavelength conversion element for converting a wavelength of a light into another wavelength using a nonlinear optical effect, and more particularly to an improvement of the QPM (Quasi-Phase Matching) structure of a wavelength conversion element.
2. Description of Related Art
Wavelength conversion elements for converting a light wavelength using an optical effect have been known. Such wavelength conversion elements are disclosed, for example, in Japanese Patent Application Laid-Open No. H5-273623 (U.S. Pat. No. 5,357,533) and Japanese Patent Application Laid-Open No. 2004-20870 (U.S. Pat. No. 6,806,986).
A wavelength conversion element is formed on a substrate having a nonlinear optical effect. For a substrate material, a z-plate of LiNbO3 can be used. The LiNbO3 substrate is a ferroelectric substance, and so it has dielectric polarization. Therefore on the LiNbO3 substrate, regions of which dielectric polarization are inverted from each other (hereafter called “first and second polarization regions”) can be formed alternately. An element structure, in which the first and second polarization regions are alternately formed, are called a QPM (Quasi-Phase Matching) structure. An optical wave guide is also formed on the substrate. A signal light and pump light multiplexed by an optical coupler are guided into the optical wave guide. An intermediate light is generated in the optical wave guide owing to the Second Harmonic Generation (SGH) of the signal light. A conversion light is also generated owing to the Difference Frequency Generation (DFG) of this intermediate light and pump light. For example, in the case when the wavelength of the signal light is 1550 nm and the wavelength of the pump light is 1540-1560 nm, the wavelength of the intermediate light is 775 nm. As a result, the wavelength of the conversion light becomes 1560-1540 nm according to the wavelength of the pump light.
The frequency band for which wavelength is converted by the wavelength conversion element changes according to conditions, such as a length of the wave guide. If the frequency band of the wavelength conversion is narrow, a high-speed pulse signal cannot be generated when the intermediate light is generated owing to SHG, and the wavelength conversion element cannot be used in a wide wavelength range. To solve these problems, a QPM structure, in which the cycles of the first and second polarization regions are gradually changed, has been known. This structure is called a “chirp structure”.
However, a problem of the conventional chirp structure is that the fluctuation of the wavelength conversion efficiency with respect to frequency is large, and therefore ripples of the pulse waveforms cannot be suppressed sufficiently. In order to suppress the fluctuation of the wavelength conversion efficiency and distortion of the pulse waveform, for the first and second polarization regions must adopt a structure in which the nonlinear optical coefficient continuously changes at both ends of the element in addition to the above mentioned chirp structure. This structure has been implemented by controlling the widths of the first and second polarization regions. This technology was disclosed in “Proceedings of 2006 Fall JSAP General Conference, 30p-ZX-12”.
However if the technology disclosed in this document is used, the dimensions of a very complicated structure must be controlled at high precision. If the dimensional accuracy is insufficient, the fluctuation of the wavelength conversion efficiency and the ripples of the pulse waveforms cannot be suppressed sufficiently. In order to implement a sufficient dimensional accuracy, the positions and dimensions of the polarization regions or electrodes for forming the polarization regions and the voltage application conditions must be controlled at high precision. But control of positions and dimensions of the electrodes have limitation because of the limit of the resolution of the electron beam writing device and the polarization region creation conditions.